1. Field of the Invention
The present invention generally relates to optical measurements and, more specifically, to a method of and apparatus for determining the resolution performance of imaging system components by measuring spatial frequency parameters. A measure of resolution of an imaging system provides an objective measure of the image quality of the imaging system.
2. Description of the Prior Art
The present invention is concerned with defining and measuring parameters representative of image resolution and is particularly adapted to matrix color and monochrome displays. While components in imaging systems are judged by the quality of a delivered image, no method or apparatus exists which accurately measures the properties associated with a high quality image.
Since directional resolution singularities and Moire' patterning within image structures are common, a method for characterizing actual imaging limitations of image system components is needed. Preferably, the metrics developed to characterize imaging properties would be measurable using appropriate laboratory apparatus. This invention introduces apparatus and methods that utilize a set of primitive image inputs whose outputs provide resolution measures of an imaging system or component.
This is accomplished through appropriate choice of image primitive and measurement in the spatial frequency domain.
The present invention creates a representation of spatial frequency power spectra from static spatial images. The spatial image to power spectral density conversion is accomplished via an optical, two-dimensional Fourier transform of the input image.
The present invention is particularly applied to the measurement of resolution for active matrix flat panel displays, and can readily be adapted to any component of an imaging system, such as the symbol generator, transmission system, or sensor.
Every imaging system component displays, processes, transmits, stores or generates images. The term "image", as we are using it here, is defined to be a continuous two-dimensional visual representation of something. As a consequence of the sampling process to be described, for all imaging system components other than the display panel the data which represents the image is not actually continuous two-dimensional visual information. Instead, the image is represented in translatable form which defines the continuous two-dimensional visual information. The visual form of the information is always a spatial distribution of light over a particular size plane.
For many display systems it is difficult for the system designer to identify the specific component or subassembly in the display system, e.g., the display head, symbol generator, transmission system, and sensors, that impairs the intended image, and to what degree the intended image was degraded or improved by implementing a design change. Heretofore the subject of image quality was based upon subjective measures, as visually observed by the operator.
Image quality has historically been a subjective entity measurable only through psychophysical experimentation and statistical analysis of many observers' opinions (c.f. L. A. Nelson, R. M. Maner, M. J. Lengyel, M. Seo, Measures of Image Quality, Society for Information Display International Symposium Digest of Technical Papers, pp 768-771, 1991). The psychophysical measures are extremely complex because chromatic, temporal, and luminance errors all contribute to perception of image quality. As these contributions are not readily measured in practice, comparisons between overall image quality determinations has heretofore not been feasible. For the foregoing reasons, a need remains in the art for an apparatus and method of objectively determining the resolution of display devices and imaging systems that is essentially independent of subjective influences.
Resolution has conventionally been defined in terms of the ability to distinguish information in the output image when a particular input target (such as the pairs of bars on a resolution chart) is used to stimulate the imaging system. The closer the spacing of pairs of bars which can be distinguished, the greater the relative resolution.
For a color Cathode Ray Tube (CRT), resolution is most often described in terms of the number of distinct dots which can be visibly displayed in a given screen area. A typical display can provide a resolution of about 100 dots per inch, indicating that dots 1/100 inch in separation can be visibly distinguished.
The methods and systems used in measuring resolution for CRT-type displays are not adequate for measuring the resolution of active color matrix displays or for any other imaging system component. The present invention utilizes the measured Fourier transform of the impulse response of the imaging system component to determine its resolving capability. This invention provides the apparatus and methodology to predict resolution accurately, via use of an image simulator, for any imaging system component before construction and to measure resolution after the component has been constructed.
The methods used in the prior art to measure resolution of CRT displays are widely varied and well documented; however, one common aspect is that the measurement is made in the spatial domain with a photometer directly from the light energy presented on the face of the display. Generally, the data is measured for only one or two (horizontal and vertical) spatial orientations, which is an incomplete characterization of the display system.
One known method requires that an electrical impulse be applied to the cathode and a light measuring instrument records the illumination footprint on the display surface. After the transient data is acquired, it is necessary to compute a Fourier transform to express the system response in the frequency domain.
Another common technique for determining the impulse response of the CRT display is to write lines at different orientations on the display and measure their cross-sectional profile with a slit scan photometer. The cross section of the line is the system's impulse response in one spatial orientation. A complete two-dimensional measure of resolution is obtained by compiling a family of transient responses. One transient response (i.e., cross-sectional profile of the line) is measured for each orientation of the line on the display surface in the spatial domain.
Another prior art method for color shadow mask CRT displays is to focus a light measuring instrument through one hole of the shadow mask. The deflection circuitry is then stimulated to scan the electron beam across the hole location on the shadow mask. The recorded illumination footprint is the impulse response (i.e., profile of the scan line) of the CRT display. Since a display system of this type generally exhibits or is assumed to be circularly symmetric, due to the nature of the electron beam, it is necessary to make the measurement in only one spatial orientation.
In E. F. Brown, et al, U.S. Pat. No. 3,657,550, there is disclosed an apparatus for measuring the spatial response of optical systems (e.g., a television system). A display is generated in a cathode ray tube by means for varying the periodicity of a predetermined spatial waveform image. The optical system under test is disposed between the cathode ray tube and a masked aperture, with a photo-detector disposed behind the mask so as to provide an output proportional to the light intensity as the spatial waveform is slowly scanned with respect to the aperture. Brown et al does not teach the use of an optical Fourier transform to resolve the spatial image into spatial frequency components of varying amplitude as in the present invention, nor does he teach a method of measurement that produces accurate results in the presence of spatial quantization of an image.
Prior art methods using spatial domain measurements are severely limited when applied to active matrix flat panel displays. FIG. 1 shows such a panel which is comprised of rectangular shaped picture elements, where R, G, and B respectively denote red, green, and blue color picture elements Such displays do not have a circularly symmetric response to a narrow line image (herein referred to as "impulse" response), nor do they have deflection circuitry. For these reasons, it is necessary to measure a family of cross-sectional profiles of line images at many orientations on the display surface to obtain a measurement of the component's resolution. This infers use of the cross-sectional slit scanned photometric method described above for CRT displays.
Erroneous measurements can occur using the slit scan methodology as a direct result of the spatial quantization (i.e, feature size of each light emitting element) of the display surface associated with color matrix displays. The physical size of the spatial quantization on the display surface dictates the length and width requirements for the slit aperture of the slit scanned photometer.
Thus, for certain geometric displays, such as an inclined line, there may result a "staircase" effect due to the finite spacing and orientation of the illuminated elements. For any orientation of the line image on a sampled display surface, the slit must be physically long enough to cover a sufficient number of display elements of the staircase such that an accurate measure of the width of the sampled line is possible. A sufficient number of such elements is captured when the staircase pattern has repeated itself either many times or exactly one time within the length of the slit aperture. FIG. 2 illustrates this limitation associated with the finite length of the slit aperture. The width of the slit aperture is the dominating factor for the spatial bandwidth of the photometric slit scan measurement system. Consequently, while maintaining the required length of the slit aperture, it is also necessary to ensure the width of the slit aperture is approximately one-tenth the pixel aperture size on the matrix display surface being measured.
The limitations associated with the slit scan method do not prohibit the use of this method for measuring the resolution of a sampled imaging system component. However, it could require a complete family of slit apertures for each measurement and possibly another family of slit apertures for displays with different pixel densities. The large number of slit apertures and measurements required for this method of measurement is extremely cumbersome and expensive to implement.
Further, the physical limitations (i.e., length-to-width ratio) associated with manufacturing the required variations of slit apertures are prohibitive, which makes this method of measurement impractical to implement in general for sampled display surfaces.
Alternative methods using a scanning slit aperture have been proposed. These methods are mechanically complex and slow in operation. They are, as a result, prone to error and involve complex manipulations of the measurement data sets after measurement. For the foregoing reasons the methods of the prior art cannot reasonably be extended beyond stroke written CRT displays.
It is known that an image may be characterized by an optical Fourier transformation of the image into a spatial light distribution pattern in which the light intensity varies in accordance with the amplitudes of the frequency components in the input signal, analogous to the Fourier transform of a complex electrical waveform into a plurality of sine waves. See, for example, J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill, 1968, pp. 141-149. See also copending U.S. Pat. Ser. No. 08/263319. Method and Apparatus for Measurement of Spatial Signal and Noise Power of Imaging Systems, filed Jun. 24, 1994, and assigned to the assignee of the present invention.
The present invention overcomes the disadvantages of the prior art by making measurements in the spatial frequency domain, not in the spatial domain. The invention is applicable to any imaging system component, and is not limited to the display device. The present invention enables a generic measurement method of resolution for any component of an imaging system. This invention is capable of generating and measuring the two dimensional spatial frequency power spectrum of its input image. When the input image is the result of an applied impulse to the imaging system component, the spatial frequency power spectrum contains the necessary information to measure resolution of the imaging system component.